Many of the forms come with outdated instructions that can trip you up. In particular, one item keeps confusing people. Never send any original documents to USCIS, unless explicitly requested by an officer (usually in an RFE). There are very few exceptions to this - of course, the forms themselves do need to be originals for instance. At some point in the past, it used to be INS policy to require and return originals, but today, they will not usually return any documents (they are way too overloaded for that!) Regular photocopies are perfectly acceptable, they don't even need to be certified.

USCIS has a Web site with more up to date information on this, and on how to get documents back that you sent in original but were not supposed to, at http://www.bcis.gov/graphics/howdoi/hdiorig.htm.

The second important thing to remember is to keep copies of everything you mail off. Mail your application using certified mail, return receipt requested (except for hand-carried applications, of course) and be sure to keep both the receipt and the copy of everything at least until the application is approved, and preferably for the rest of your life.